The controller area network and an expansion of the CAN referred to as a time-triggered CAN (TTCAN) are known from the published patent application DE 100 00 305 A1. The media access control method used in the CAN is based on bitwise arbitration. In bitwise arbitration, several user stations may transmit data simultaneously over the channel of the bus system without this interfering with the data transmission. Upon sending a bit over the channel, the user stations may also ascertain the logic state (0 or 1) of the channel. If a value of the sent bit does not correspond to the ascertained logic state of the channel, then the user station terminates the access to the channel. In the CAN, the bitwise arbitration is normally carried out in an arbitration field within a data frame to be transmitted over the channel.
After a user station has completely sent the arbitration field to the channel, it knows that it has exclusive access to the channel. Thus, the end of the transmission of the arbitration field corresponds to a beginning of an available interval, within which the user station may use the channel exclusively. According to the protocol specification of the CAN, other user stations may not access the channel, that is, send data to the channel, until the sending user station has transmitted a checksum field (CRC field) of the data frame. Thus, an end time of the transmission of the CRC field corresponds to an end of the available interval.
The bitwise arbitration allows non-destructive transmission of the data frame over the channel to be achieved. This produces effective real-time characteristics of the CAN, whereas the media access control methods, in which the data frame sent by a user station may be destroyed, due to a collision with a further data frame sent by another station during the transmission over the channel, have a markedly poorer real-time performance, since the collision and the re-transmission of the data frame rendered necessary by it result in a delay in the data transmission.
The protocols of the CAN and of its expansion, TTCAN, are particularly suited for transmitting short messages under real-time conditions. However, if larger data blocks are to be transmitted over a CAN domain, then the relatively low bit rate of the channel becomes a limiting factor. In order to ensure the correct functioning of the bitwise arbitration, a minimum time period dependent on, in particular, the extent of the bus system, the signal propagation speed on the channel, and intrinsic processing times in the interface modules of the bus users, must be maintained for the transmission of a bit, since during the arbitration, all of the bus users must have a uniform picture of the bus state (0 or 1) and equal access to the bus state. Thus, the bit rate cannot easily be increased by reducing the duration of the individual bits.
Nevertheless, in order to be able to transmit sufficiently rapidly a relatively large data block, which is necessary for the programming of a control unit, via a communications interface actually provided for connecting to a CAN domain, DE 101 53 085 A1 provides that for transmitting the data block, the communications interface be temporarily switched over into a different communications mode, in which bitwise arbitration is not carried out and, therefore, a relatively high bit rate is possible. However, in this connection, the communication with the protocols of the CAN must be interrupted for a certain amount of time. If, e.g., due to an error, the operation of the bus system cannot be initiated in accordance with the CAN protocols, then the bus system will stop functioning. In addition, due to the transmission of a relatively large data block, the subsequent transmissions to be carried out according to the CAN protocols are considerably delayed, which means that the real-time characteristics of the CAN are seriously degraded. Therefore, it is not practical to use this method both for programming the control unit at the end of a manufacturing process of a motor vehicle or of the control unit, and during operation of the vehicle.
German patent document DE 103 11 395 A1 discusses a system, which may be switched over between CAN communication and an asymmetric, serial communications protocol and allows a higher transmission rate in the asynchronous mode. However, this protocol is outside of the CAN standard.
German patent document DE 103 40 165 A1 discusses improved synchronization between sensors and actuators within a CAN network. This reduces the latency times, but the transmission rate is not increased.